While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-ultraviolet lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using a KrF or ArF excimer laser as the light source is strongly desired to reach the practical level as the micropatterning technique capable of achieving a feature size of 0.3 μm or less.
The resist materials for use in photolithography using light of an excimer laser, especially ArF excimer laser having a wavelength of 193 nm, are, of course, required to have a high transparency to light of that wavelength. In addition, they are required to have an etching resistance sufficient to allow for film thickness reduction, a high sensitivity sufficient to eliminate any extra burden on the expensive optical material, and especially, a high resolution sufficient to form a precise micropattern. To meet these requirements, it is crucial to develop a base resin having a high transparency, rigidity and reactivity. None of the currently available polymers satisfy all of these requirements. There exists a strong demand to further improve resist materials.
Most common among high transparency resins used in the art are copolymers of acrylic or methacrylic acid derivatives. Active research works have been made on these copolymers to use them effectively as resist material. JP-A 4-39665 describes that an alicyclic structure such as adamantyl must be introduced as an ester substituent group into copolymers of acrylic or methacrylic acid derivatives in order to enhance the structural rigidity of the backbone and to provide necessary etching resistance. The introduction of an alicyclic structure, on the other hand, increases the water repellency of polymers, which disturbs smooth development with an aqueous tetramethylammonium hydroxide solution, a developer, tending to have deleterious effects of forming micro-patterns with configuration defects and eventually incurring pattern disruption. When these polymers are formulated as the base resin into resist compositions, the resulting resist compositions in some cases have satisfactory resolution, but do not withstand etching, and in other cases, have acceptable etch resistance, but a low resolution and insufficient performance.